Liquid crystal compounds have been widely used in display devices since the 1970's. Liquid crystal displays have many outstanding advantages in, for example, that they can be made to exhibit thin-thickness and light-weight, require low driving voltage and low power consumption, and are non-emissive. Because of these advantages, liquid crystal displays are widely considered as the mainstream display devices of the next generation.
Most of the conventional display devices are used in a TN-type display system, and nematic liquid crystal materials are most commonly used for this purpose. However, the TN display system has several limitations in the areas of response time and the width of the viewing angle. Therefore, in order to broaden the acceptance of liquid crystal display devices and utilize the advantages thereof as illustrated hereinabove, development of improved liquid crystal material as an alternative to the nematic phase type liquid crystal is essential.
Recently, ferroelectric liquid crystals have received significant attention as an alternative to the nematic type liquid crystals. In the past decade and half, over 500 patents have been issued in the areas related to ferroelectric liquid crystals. The existence of ferroelectric liquid crystals was first observed in 1974 which was subsequently published in 1975 by R. B. Meyer, L. Liebert, L. Strzelecki, and P. Keller (see J. Physique Letters, 1975, 36, L-69). They reported that strong ferroelectricity could be observed from liquid crystal belonging to a chiral smectic C phase (Sc* phase). They also synthesized a liquid crystal compound (S)-4-n-deecyloxybenzylideneamiao-2'-methylbutyl cinnamate (DOBAMBC) to prove their theory. In 1980, N. A. Clark and S. T. Lagerwall proposed a liquid crystal display system wherein an optical switching phenomenon of a ferroelectric liquid crystal was utilized. (See Appl. Phys. Lett., 1980, 36, 899; see, also, U.S. Pat. No. 4,367,924, entitled "Chiral Smectic C or H Liquid Crystal Electro-Optical Device"). The discovery of Clank and Lagerwall opened the door for ferroelectric liquid crystals to be used in practical applications.
Ferroelectric liquid crystals are known to have bistability. By definition, liquid crystals with bistability are those molecules which are horizontally oriented with respect to the electrode surface as stable state and are vertically oriented with respect to the electrode surface only when an electric field is effectively applied. Ferroelectric liquid crystals have bistable states of first and second stable states with respect to an electric field applied thereto. Accordingly, the bistable ferroelectric liquid crystals are oriented to the first and second optically stable states with respect to one and the other of the electric field vectors, respectively. Furthermore, ferroelectric liquid crystals have also shown to provide high-speed switching characteristic. This is because the high spontaneous polarization of the ferroelectric liquid crystals and an externally applied electric field directly interact with each other to induce transition of orientation states.
Ferroelectric liquid crystals also exhibit excellent characteristics of having high contrast and wide viewing angle, and thus are particularly suitable for use in large-sized displays using a simple matrix. These advantages, coupled with their bistability and high-speed switching characteristic, accord an excellent commercial potential for ferroelectric liquid crystals, especially in the area of flat-panel displays.
Ferroelectric liquid crystals belong to the family of tilted smectics; however, the most important ferroelectric liquid crystals belong to the chiral smectic C phase. In practical applications, a mixture of ferroelectric liquid crystals of various types, rather than a single type, are used. For those ferroelectric liquid crystals that do not exhibit the chiral smectic C phase, a chiral dopant can be added to a liquid crystal host. (See W. Kuczynski, H. Stegemeyer, Chem. Phys. Lett., 1980, 70, 123; S. M. Kelly, A. Villiger, Displays, 1990, 41.)